Method of preparing zirconium carbide fibers and the product thereof



United States Patent 3,385,669 METHOD OF PREPARING ZHRCONIUM CARBIDEFIBERS AND THE PRODUCT THEREOF Robert A. Clifton, .Ir., Knoxville, andRobert C. Johnson,

Norris, Tenn, assignors to the United States of Amerion as representedby the Secretary of the Interior No Drawing. Filed Apr. 21, 1965, Ser.No. 449,898 4 Claims. (Cl. 23-208) ABSTRACT OF THE DISCLOSURE Zirconiumcarbide fibers are produced by reacting zirconium oxide fibers withcarbon.

The invention herein described and claimed may be manufactured and usedby or for the Government of the United States of America forgovernmental purposes without the payment of royalties thereon ortherefor.

This invention relates to preparation of inorganic refractory carbides,particularly zirconium carbide, in fibrous form. Such fibers are usefulfor any application where high temperature refractory fibers are needed,in n-onoxidizing atmospheres. Examples are reinforcement for hightemperature composites, loose fill thermal insulation and feltedinsulation.

Oopending application Ser. No. 438,130 discloses preparation of fibersof inorganic refractory materials such as zirconia. It has now beenfound that the fibers prepared by the method of this copendingapplication can be converted to the corresponding carbide, in fibrousform, by treatment of the oxide fiber with carbon at elevatedtemperature.

Conversion of the oxide to carbide involves reduction and dififusion atthe elevated temperature. Lampblack has been found to be very effectivein the process and is employed in the example below; however, othercarbon materials such as graphite or coke fines may be used. The oxidefiber is intimately contacted with the carbon as by embedding in thelatter and the mixture is then fired at a suitable temperature for atime sufiicient to convert the oxide to carbide.

Optimum temperature and time of firing may vary considerably dependingon the type and size of the inorganic fiber and are best determinedexperimentally. In general, however, temperatures may range from about800 C. to about 1600 C. to 1800 C. with reaction time being from aboutminutes to several hours. Optimum amounts of carbon are also notcritical and are readily determined experimentally; however, it isgenerally necessary to provide an excess of carbon for embedding theoxide fiber.

The reaction is carried out under vacuum or inert gas atmosphere toprevent oxidation of the carbide product at the elevated temperature. Aninert atmosphere is generally preferable for continuous production ofthe carbide fibers. Gaseous CO is also formed as a product of thereaction and may be removed by any suitable means.

The products of the invention are refractory fibers of considerablyhigher melting point than the parent oxide fibers (e.g., 3175 0:50 forZrC fibers as compared to 2700 C. for ZrO fibers). They have crystalforms stable at all usable temperatures and low thermal coefiicients oflinear expansion (e.g., 6.73 10- percent/degree for ZrC fibers). Theproducts also have a very low thermal 3,3855% Patented May 28, 1968conductivity rating (0.049 caL/cm. sec. at 100 C. for ZrC). In addition,fibers of any length and with controllable diameter may be prepared byusing oxide fibers of similar dimensions as starting material.

Carbides have previously been manufactured from oxides by treatment withcarbon as disclosed, e.g., in US. Patent 3,161,470. However, preparationof polycrystalline carbide fibers having the advantageous propertiesdiscussed above has not been previously known.

The invention will be more specifically illustrated by the followingspecific example.

Example Procedure. -A small portion of the Zr0 fibers were embedded inlampblack in a graphite crucible and the cruicible put into an Ajaxinduction furnace. The furnace was evacuated to 100 microns and a slowinitial temperature rise begun. The temperature was raised to 1700 C.and held there for 15 minutes, after which the furnace was cooled andthe sample removed.

Results.-Up0n removal from the crucible all of the fibers had a lustrousgrey color. X-ray diffraction showed greater than percent cubic ZrC andno detectable amounts of ZrO The fibers were essentially unchangedphysically (i.e., a pseudomorph), but were nearly completely alteredchemically from ZrO to ZrC.

Although the invention has been specifically described with reference tothe preparation of ZrC fibers, it may also be used to prepare fibers ofother refractory carbides, such as Ti, Hf, V, Nb, Ta, Mo, and W caribdesby treat ment of the corresponding oxide fiber with carbon at elevatedtemperatures.

What is claimed is:

1. A method for preparation of fibers of Zirconium carbide comprisingreacting fibers of zirconium oxide with carbon in an inert environmentat an elevated temperature of about 1700 C.

2. A polycrystalline zirconium carbide fiber produced by the process ofclaim 1.

3. Method of claim 1 in which the reaction is carried out at saidtemperature of about 1700 C. for about 15 minutes.

4. Method of claim '3 in which the carbon is lampblack.

References Cited UNITED STATES PATENTS 2,800,393 7/ 1957 Mettler 23-2083,088,805 5/1963 Cotter et al 23208 X FOREIGN PATENTS 676,321 12/ 1963Canada.

OTHER REFERENCES Schwarzkopf et al., Refractory Hard Metals, The Mac-Millan Co., 1953, pp. 89-91. TN 677 S 36.

Burgers et al., Zeitschrift fur Anorganische und Allgemeine Chemie, 216,January 1964, pp. 217, 218 and 222. QD 1 Z 4.

OSCAR R. VERTIZ, Primary Examiner.

MILTON WEISSMAN, Examiner.

G. T. OZAKI, Assistant Examiner.

